According to an obstacle recognition technique in the field of automotive technology, an existence of an obstacle located around a vehicle is acknowledged to a driver of the vehicle by means of a sound, thereby allowing the driver to recognize the existence of the obstacle. For example, according to a known obstacle recognition technique described in JP3179227B2 (refer to Paragraphs 0014 to 0018, FIGS. 6 to 8) (hereinafter referred to as Patent document 1), the localization of a warning sound or simulated sounds such as driving sounds of other vehicles around a vehicle of a driver and a wheel slip sound of the vehicle of the driver is varied in accordance with balance adjustment of sound volumes of a plurality of speakers and in accordance with application of a delay sound in order to alert directions of other vehicles located around the vehicle to the driver. Accordingly, the driver is allowed to recognize an existence of an obstacle around the vehicle. Similarly, in addition to the localization of sounds emitted from the speakers in accordance with control for output of a delayed sound or in accordance with balance adjustment of sound volumes, according to a known obstacle recognition technique described in JP2785234B2 (refer to Paragraphs 0021 to 0026, FIG. 2) (hereinafter referred to as Patent document 2), an existence of an obstacle existing around a vehicle is acknowledged to a driver of the vehicle by means of patterns of warning sounds sorted by the location of the obstacle.
Moreover, in addition to the method to alert a driver to an existence of an obstacle around his/her vehicle by means of the above-described localization of sounds, a known method for tone assignment is described in JP1994-84079A (refer to Abstract, FIG. 6) (hereinafter referred to as Patent document 3). Tones are assigned to respective spatial positions according to the tone assignment method. An obstacle position recognition support system is equipped with a database memorizing predetermined tones for respective spatial positions and a reproducing device reproducing a tone selected based on the spatial position, so that each spatial position can be indicated by the selected tone according to the tone assignment method of Patent document 3.
However, according the obstacle recognition techniques of Patent document 1 and Patent document 2, a sound is required to be continuously emitted from the timing when an obstacle is located relatively far from the vehicle of the driver. In addition, a sound volume, a tone, an output cycle, a delay sound, and an initial reflected sound are required to be gradually varied, so that the driver perceives the variations. Accordingly, it is difficult for an existence of an obstacle such as a pedestrian unexpectedly approaching the vehicle to be notified to the driver. Further, in cases where a sound is continuously emitted, the driver may feel that the sound is noisy, or the driver accustoms himself/herself to the sound and may disadvantageously lose a sense of direction or distance relative to variations of the sound.
Moreover, according to the tone assignment method of Patent document 3, the driver is required to previously know which tone is set to which location. In addition, for example, according to such tone assignment method of Patent document 3, the tone assignment may be conducted on the basis of a consensus that a high frequency tone is emitted when an obstacle exists close to the vehicle of the driver. However, disadvantageously, the driver may not always perceive the existence of the obstacle on the basis of such consensus. Since various kinds of sounds are assigned in order to cover spatial positions, the driver may be confused when an obstacle unexpectedly appears close to the vehicle.
A need thus exists for a surrounding recognition support system, which is not susceptible to the drawbacks mentioned above.